a) Field of the Invention
The present invention relates to a variable magnification viewfinder optical system to be used in cameras, etc., and more specifically to a variable magnification viewfinder optical system suited for use in 35 mm lens shutter cameras.
b) Description of the Prior Art
In the recent years, there have been proposed numerous 35 mm lens shutter cameras which are adapted to be used at two selectable focal lengths. For such cameras having two selectable focal lengths, it is convenient for confirming actual photographing field angles on finders to compose finders so as to permit varying magnifications thereof in conjunction with the selection of focal lengths of the photographic lens systems. For providing this convenience, there have already been proposed a variety of finder optical systems which permit varying magnifications thereof. For example, Japanese Preliminary Patent Publication No. Sho 60-166934 proposed an inverted Galileo finder comprising a negative objective lens component and a positive eyepiece lens component arranged in the order from the object side, and adapted to permit varying magnification of the finder by replacing said negative objective lens component with another objective lens component which is composed of a positive lens element and a negative lens element arranged in the order from the object side.
Further, Japanese Preliminary Patent Publication No. Sho 61-87122 proposed a finder optical system comprising, in the order from the object side, a fixed lens component having a positive refractive power as a whole, a negative movable lens component and a fixed lens component having a positive refractive power as a whole, and adapted to vary focal length of the finder by moving said negative movable lens component forward and backward along the optical axis.
Furthermore, Japanese Preliminary Patent Publication No. Sho 63-129312 proposed an inverted Galileo finder which comprises, in the order from the object side, a negative objective lens component composed of two negative lens elements and a positive fixed eyepiece lens component, and is adapted a to vary magnification thereof by placing one of the two negative lens elements out of the effective optical path of the finder and moving the other negative lens element toward the eyepiece lens component along the optical axis.
Japanese Preliminary Patent Publication No. Sho 61-77820 proposed a finder optical system which comprises an objective lens component designed as a negative lens component consisting of a lens element having a positive refractive power and a lens element having a negative refractive power including at least one lens element made of a transparent elastic material, and an eyepiece lens component designed as a positive lens component, and is adapted to vary magnification thereof by deforming the transparent elastic material lens element so as to vary the refractive power of the objective lens component as a whole.
Japanese Preliminary Patent Publication No. Sho 61-221720 proposed a finder optical system which comprises an objective lens component and an eyepiece lens component each composed of a single lens element made of a transparent elastic material, and is adapted to vary magnification thereof by deforming the lens elements so as to vary the refractive powers thereof while maintaining a certain definite relationship therebetween.
Japanese Preliminary Patent Publication No. Sho 62-56918 proposed a finder optical system which comprises lens elements including ones designed as liquid crystal lens elements and is adapted to vary magnification thereof by moving some of the lens elements other than the liquid crystal lens elements in accordance with variation of the refractive powers of the liquid crystal lens elements.
Out of the conventional examples described above, however, each of the finder optical systems which is adapted to vary the magnification thereof by moving a portion of the optical system has a composition to move one or more lens elements to vary the refractive power of the objective lens component as a whole, and therefore has disadvantages such as the mechanism for varying the magnification is complicated and requires high precision, and cost is increased for manufacturing the finder optical system in practice since the number of parts is increased, thus requiring tedious assembly procedures and delicate adjustment. Speaking concretely, it is necessary to compose the magnification varying mechanism in such a manner that the movable lens elements are located with correct distances reserved to the other fixed lens elements before and after the movements and with no eccentricities with regard to the optical axis. Otherwise, it will be difficult to attain to the design performance such as the magnification, dioptric power, field ratio, and image legibility. Further, the finder optical systems which are adapted to vary magnifications thereof by using the lens elements having the variable refractive powers, out of the conventional examples described above, control the refractive powers by deforming the external shapes of the lens elements having the variable refractive powers under physical forces of driving devices and must control the driving devices with high accuracy so as to obtain stable deformation degrees, thereby having disadvantages that the driving mechanisms and the control circuits therefor are complicated and that costs are enhanced for manufacturing the finder optical systems in practice.